If anyone is interested, I took photos & can add.

Thanks for the details on what the observed features really were.
Water coming out the pipe is clear; then it hits the pool. And the dog didn't seem to mind, though I did get a chuckle when it tried to jump the deepest spot and failed. This weekend I had the cairn with me. Very short legs, but happy to try anything- including swimming out to sample some feed.

Bingo Snrub. Piping is what it is, though I am fortunate to have a drain to play with. Aeration can be done with some work. Just have to decide if next year when 2-3" predators go in, or can it wait to the next year when they'll get more size.